1. Field of the Invention
The present invention relates to nickel-base single-crystal superalloys applied to parts for industrial high temperature gas turbine, such as turbine rotor blade and stator blade, a method for manufacturing the same, and gas turbine parts prepared therefrom.
2. Description of the Related Art
With a trend towards high efficiency of gas turbine, combustion temperature therefor rises, so that turbine rotor and stator blades have changed from a type of ordinary cast blade to a type of unidirectional solidified blade wherein crystal grain boundary along the stress axial direction is removed to improve creep strength at high temperatures, and further to a type of single-crystal blade wherein grain boundary reinforcing elements the presence of which is a cause for decreasing heat treatment characteristics are excluded by allowing crystal grain boundary to disappear, so that the optimum heat treatment is applied to elevate a ratio of deposition of .gamma.', whereby creep strength at high temperatures are more improved.
On the other hand, materials applied to single-crystal blade have been improving in creep strength.
A first generation single-crystal alloys contain no Re (Renium). As an example of such alloys, Japanese Patent Application Laid-Open No.19032/1984 discloses CMSX-2. U.S. Pat. No. 5,399,313 discloses Rene' N4. Japanese Patent Application Laid-Open No.146223/1978 discloses PWA-1480.
Second generation single-crystal alloys contains around 3% of Re, and so its creep rupture strength is about 30.degree. C. better than that of the first generation single-crystal alloys. As an example of second generation single-crystal alloys, U.S. Pat. No. 4,643,782 discloses CMSX-4. U.S. Pat. No. 4,719,080 discloses PWA-1484, and Japanese Patent Application Laid-Open No. 59474/1993 discloses Rene' N5.
Furthermore, third generation single-crystal alloys contain around 5% to 6% of Re, and as an example of such alloys, CMSX-10 is disclosed in Japanese Patent Application Laid-Open No. 138683/1995.
Although these single-crystal alloys have been developed remarkably in mainly fields of aircraft jet engines and small gas turbines, it has been intended to convert such technology therefor into also a field of large gas turbines for industrial use because of achieving high temperatures directing to improvements in combustion efficiency.
When a third generation single-crystal alloy is applied, high temperature strength can be obtained, while there rises a disadvantage of poor structural stability. Particularly, since an industrial large gas turbine takes longer time for design life as compared with aircraft jet engine or small gas turbine, it is required as to blade materials to inhibit formation of TCP (Topoligically Close-Packed) phase, i.e., to attain good structural stability. Therefore, it is not suitable to apply such a third generation single-crystal alloy to a large gas turbine.
For example, by adding further 5% to 6% of Re, a third generation single-crystal alloy can achieve a higher strength than that of a second generation single-crystal alloy. However, when such a third generation single-crystal alloy is adopted for a long period of time, TCP phase which becomes a starting point of low cycle rupture is produced, so that creep rupture arises, because such a third generation alloy is added 5% to 6% of Re.
In addition, Re decreases a casting yield in case of manufacturing a large gas turbine due to reaction with its casting mold, and it deteriorates also heat treatment characteristics due to segregation. In these circumstances, a reacting area with a casting mold increases, and its heat treatment is required for a long period of time in gas turbine for industrial use wherein a blade dimension becomes larger than that of aircraft gas turbine.
Therefore, nickel-base superalloys containing around 3% of Re, that is, a second generation single-crystal alloys, is studied as a material of gas turbine rotor and stator for large generator at present. And so, it is desirable that gas turbine is improved in creep strength, because of need for much higher combustion temperature.
In comparison with the characteristic property required for next generation gas turbine rotor and stator blades, second generation single-crystal alloy exhibits usually a sufficient creep strength at a temperature of 900.degree. C. or lower and in a stress of 200 MPa or more, while there is a problem of an insufficient creep strength at a high temperature of more than 900.degree. C. and in a stress of less than 200 MPa.